In recent years, electronic equipment in which semiconductor devices are mounted has been used in a wider range of applications, or under severe conditions or environments, such as vibrations, temperature cycles, stresses, and various atmospheres, which require high reliability of the electronic equipment. To keep up with the expansion of the use of electronic equipment, it is extremely important to provide semiconductor devices having sufficiently high reliability. In particular, it has been strongly desired to improve the reliability of connecting portions at which the semiconductor devices and external components or devices are electrically joined to each other. Known methods for electrically connecting the semiconductor devices with the external components include wire bonding, soldering, and pressure welding, for example.
Among the above-indicated connecting methods, soldering is used in many applications since it enables a large number of connecting portions to be joined together at a time at a relatively low temperature. FIG. 5(a) is a cross-sectional view showing an electrode portion of a semiconductor device prior to soldering.
An electrode consisting of an alloy layer (hereinafter referred to as "Al layer") 2 having a thickness of 0.5 .mu.m and containing aluminum as a major component, a nickel (Ni) layer 3 having a thickness of 0.5 .mu.m, and a gold (Au) layer 4 having a thickness of 0.2 .mu.m is formed on a semiconductor substrate 1 made of silicon (Si), by electron beam deposition or other method. The Ni layer 3 is used for soldering, and the Au layer 4 is provided for preventing oxidation of the electrode surface and improving wettability.
FIG. 5(b) is an enlarged cross-sectional view showing the electrode portion after soldering. A terminal 6 is joined to the Ni layer 3 with a lead-tin solder 5 interposed therebetween. After actually using the semiconductor device thus obtained by soldering, it was found that part of its electrode suffered from considerable reduction in the joining strength.
FIG. 5(c) is an enlarged cross-sectional view showing the electrode portion in which the considerable reduction in the joining strength was observed.
In the electrode portion of FIG. 5(c), an aluminum-nickel (Al--Ni) intermetallic compound 7 developed or grew from the interface between the Al layer 2 and the Ni layer 3, down to the interface between the Al layer 2 and the Si substrate 1, and clearances 8 were formed between the Al--Ni intermetallic compound 7 and the Si substrate 1. Observing the electrode portion of FIG. 5(b) more carefully, it was also found that an Al--Ni intermetallic compound 7 was formed between the Al layer 2 and the Ni layer 3.
Since the electrode is subjected to high temperatures in the range of 300 to 500.degree. C. during lamination of the layers and soldering, and also under conditions of the use thereof, the Al--Ni intermetallic compound 7 grows from the interface between the Al layer 2 and the Ni layer 3, and the clearances 8 are formed probably because of volume changes (volume decrease) due to phase changes from the Al layer 2 and Ni layer 3 to the Al--Ni intermetallic compound 7. As a result, the joining strength between the terminal 6 and the Si substrate 1 is rapidly reduced.
In addition to the high temperature as described above, electrodes of semiconductor devices are generally exposed to other thermal stresses caused by, for example, heating of the Si substrate when Al layer or Ni layer is formed thereon, heat treatment for removing defects in the Si substrate through electron beam irradiation, heat treatment for removing water vapor adhering to the surface, abnormal high temperatures due to temperature variations within the device during the above heat treatments, abnormal high temperatures due to temperature variations among lots during the heat treatment processes, and heat generated it local portions of the semiconductor device due to concentration of current during the use of the device.